(a) Technical Field
The present disclosure relates, in general, to a separator for a fuel cell. More particularly, it relates to a carbon fiber reinforced composite separator for a polymer electrolyte membrane fuel cell and a method for manufacturing the same.
(b) Background Art
In general, a polymer electrolyte membrane fuel cell (PEMFC) is a device that generates electricity with heat and water by an electrochemical reaction between hydrogen and oxygen (or air) as reactant gases. The PEMFC has certain advantages such as high energy efficiency, high current density, high power density, short start-up time, and rapid response to a load change as compared to the other types of fuel cells. Accordingly, it can be used in various applications such as a power source for zero-emission vehicles, an independent power plant, a portable power source, a military power source, etc.
The configuration of a fuel cell stack is described with reference to FIG. 1 below.
In a typical fuel cell stack, a membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack. The MEA comprises a solid polymer electrolyte membrane 60, through which hydrogen ions (protons) are transported, and catalyst layers 61 including a cathode and an anode, which are coated on both surfaces of the electrolyte membrane 60.
Preferably, a gas diffusion layer (GDL) 40 and a gasket 41 are sequentially stacked on the outside of the electrolyte membrane 10, i.e., on the surface where the cathode and the anode are positioned. A separator (also called a bipolar plate) 30 including flow fields for supplying fuel and discharging water produced by the reaction is stacked on the outside of the GDL 40. Further, end plates 50 for supporting the above-described elements are suitably connected to the both outermost ends.
An oxidation reaction of hydrogen occurs at the anode of the fuel cell to produce hydrogen ions (protons) and electrons, and the produced hydrogen ions and electrons are transmitted to the cathode through the electrolyte membrane and the separator, respectively. At the cathode, the hydrogen ions and electrons transmitted from the anode through the electrolyte membrane and the separator react with oxygen in air to produce water. Here, electrical energy is generated by the flow of the electrons through an external conducting wire due to the transfer of the hydrogen ions, and at this time, heat and water are produced by the electrochemical reaction.
In the above-described fuel cell stack, the separator separates the respective unit cells of the fuel cell and, at the same time, serves as a current path between the unit cells, and the flow fields formed in the separator serve as paths for supplying hydrogen and oxygen and discharging water produced by the reaction.
Since the water produced by the reaction inhibits the chemical reaction occurring on the catalyst layers of the fuel cell, the water should be rapidly discharged to the outside, and therefore the separator material may have high surface energy such that the water is rapidly spread on the surface of the separator (hydrophilicity) or may have low surface energy such that the water rolls down the surface of the separator (hydrophobicity).
In particular, it is necessary to minimize the electrical contact resistance between the separators. Conventionally, the separator is formed of graphite, thin stainless steel, or a composite material in which expanded carbon particles or graphite particles are mixed with a polymer matrix. Recently, an attempt to prepare a composite separator using continuous carbon fibers has been made.
Accordingly, research aimed at developing a continuous carbon fiber composite separator which can improve the electrical, chemical, and mechanical properties has continued to progress, and a method for reducing the electrical contact resistance between the unit cells, which is an important electrical property, has been studied.
Korean Patent Publication No. 10-2009-0112771, incorporated by reference in its entirety herein, discloses a continuous carbon fiber composite separator and a method for manufacturing the same. According to this method, the separator is manufactured using a continuous carbon fiber composite and, at this time, a hot press or hot roller having positive and negative flow field patterns is used to form hydrogen, air, and coolant flow fields, thereby manufacturing a separator having excellent impact strength, moldability, chemical resistance, and flexural strength, compared to the existing separators formed of other materials.
FIG. 2 is a process diagram showing a method for manufacturing a composite separator disclosed in the above-referenced patent. As shown in the figure, a raw material 10 of a continuous carbon-fiber composite having a continuous fiber structure, in which carbon fibers are surrounded by a polymer binder, is wound on a roll 12. Here, the raw material 10 of the continuous carbon-fiber composite is provided as a prepreg in the form of a semi-cured sheet or as a sheet molding compound (SMC) in a semi-cured state.
Next, the raw material 10 of the continuous carbon-fiber composite is passed through a plurality of cutting rollers 18 in the form of a long roll including a cutter 16 provided on the surface thereof such that the raw material 10 is cut along the length of a separator and, at the same time, a common distribution manifold and an assembly hole are formed on the raw material 10.
Subsequently, a plurality of raw materials 10 cut along the length of the separator is continuously passed through a stacking/compression roller 20 or a plurality of raw materials 10 cut along the length of the separator is arranged in a zigzag manner such as 0°/90°/0° and then passed through the stacking/compression roller 20.
Then, the stacked raw materials 10 are placed on a high temperature hot press 22 to be press-molded or passed through a high temperature hot roller to be press-molded, heated, and partially cured.
Here, positive and negative flow field patterns 24 for forming hydrogen, air, and coolant flow fields are provided on the surface of the high temperature hot press 22 and the high temperature hot roller. Accordingly, the hydrogen, air, and coolant flow fields are formed on the raw material 10 by the press molding of the high temperature hot press 22 or the high temperature hot roller.
Next, a trimming process is performed to remove unnecessary portions from the heated and press-molded separator using a trim cutter 26. The trimming process is performed within a minimum period of time so that the heated and press-molded raw material 10 has a curing degree that can maintain its shape.
Lastly, a post-curing process, in which several hundreds of separators are placed in an autoclave at a time to be heat-treated, is performed so that the separators are finally cured.
Therefore, according to the above-described method, it is possible to manufacture the continuous carbon fiber composite separator by a continuous process for mass production, and it is possible to provide a separator having excellent mechanical and chemical resistance properties.
However, residual resin remains on the surface of the continuous carbon fiber composite separator manufactured in the above-described manner, and it may increase the electrical contact resistance between the composite separator and GDL, thereby reducing the efficiency of the fuel cell due to ohmic loss.
Moreover, a process for bonding a gasket for maintaining airtightness to the thus manufactured composite separator is an obstacle to produce it because the process may either increase processing time or thermally damage the composite separator.
That is, in the case where a solid gasket is bonded to the separator using adhesive, the processing time is increased, and this results in reduced productivity. In the case where a liquid phase resin for gasket is injected onto the surface of the separator, the liquid phase resin for gasket is exposed to a temperature of 250 to 300° C. for a long time to be cured, during which the composite material of the separator may be damaged.
Accordingly, there is a need in the art for new methods for manufacturing a composite separator for a fuel cell.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.